What should be the lifeguard's first action for a passive drowning person with no pulse and in cardiac arrest?

For a passive drowning person who is in cardiac arrest, the immediate and appropriate action is to provide rescue ventilations after ensuring the scene is safe and you have assessed the situation. This is because drowning victims often experience hypoxia—an insufficient supply of oxygen to the body—including in their lungs. Rescue ventilations can help to deliver oxygen directly into the lungs, which is crucial before initiating chest compressions.

This action is particularly important for drowning cases due to the likelihood that the airway is obstructed or that there is water in the lungs, making effective ventilation essential. While starting chest compressions is vital, it should ideally follow the initial step of supplying ventilations to restore oxygenation. Subsequently, depending on circumstances, you would also progress to calling for help, as it's critical to have emergency medical assistance on the way. Checking for breathing would typically happen as part of your primary assessment, but in this instance, you are already aware of the need for immediate ventilations first.